Since the demonstration of efficient organic light emitting diodes (OLEDs) by Tang et al. in 1987 (C. W. Tang et al., Appl. Phys. Lett. 51 (12), 913 (1987)), OLEDs developed from promising candidates to high-end commercial displays. An OLED comprises a sequence of thin layers substantially made of organic materials. The layers typically have a thickness in the range of 1 nm to 5 μm. The layers are usually formed either in vacuum by means of vapor deposition or from a solution, for example by means of spinning on or printing.
OLEDs emit light after the injection of charge carriers in the form of electrons from the cathode and in form of holes from the anode into organic layers arranged in between. The charge carrier injection is effected on the basis of an applied external voltage, the subsequent formation of excitons in a light emitting zone and the radiative recombination of those excitons. At least one of the electrodes is transparent or semitransparent, in the majority of cases in the form of a transparent oxide, such as indium tin oxide (ITO), or a thin metal layer.
Flat displays based on OLEDs can be realized both as a passive matrix and as an active matrix. In the case of passive matrix displays, the image is generated by for example, the lines being successively selected and an image information item selected on the columns being represented. However, such displays are restricted to a size of approximately 100 lines for technical construction reasons.
Displays having high information content require active driving of the sub-pixels. For this purpose, each sub-pixel is driven by a circuit having transistors, a driver circuit. The transistors are usually designed as thin film transistors (TFT). Full color displays are known and typically used in mp3-players, digital photo cameras, and mobile phones; earliest devices were produced by the company Sanyo-Kodak. In this case, active matrices made of polysilicon which contain the respective driver circuit for each sub-pixel are used for OLED displays. An alternative to polysilicon is amorphous silicon, as described by J.-J. Lih et al., SID 03 Digest, page 14 et seq. 2003 and T. Tsujimura, SID 03 Digest, page 6 et seq. 2003. Another alternative is to use transistors based on organic semiconductors.
Examples of OLED layer stacks used for displays are described by Duan et al (DOI: 10.1002/adfm.201100943). Duan shows blue OLEDs and white OLEDs. He modified the devices with one light emitting layer to a double and triple light emitting layer, achieving a longer lifetime at the cost of a more complex device stack. Other state-of-the art stacks are disclosed in U.S. Pat. No. 6,878,469 B2, WO 2009/107596 A1 and US 2008/0203905.
Generally, in electronic and/or optoelectronic devices requiring charge transfer through phase interfaces, minimization of contact resistances occurring on these interfaces is required, to achieve low operating voltages, high energetic efficiency and low heat load. Charge injection layers comprising organic or inorganic electrical n- or p-dopants are known as a means allowing enhanced charge injection into adjacent semiconducting layers.
Small-molecule organic dopants that can be deposited at relatively low temperatures e.g. by vacuum thermal evaporation (VTE) and/or by solution processing like dip coating, spin coating or jet printing are already used in mass OLED and display production. Yet, there is significant disadvantage that on the interface between the organic charge injecting layer consisting of small molecules and the adjacent organic layer, poorly reproducible processes can occur during deposition of the adjacent organic layer. In certain cases, especially if the adjacent layer is deposited by solution processing, special precautions are necessary for avoiding complete destruction of the previously deposited injection layer.
Monomolecular organic layers chemically anchored to an inorganic substrate are known and studied preferentially as so called self-assembling monolayers (SAMs). A good introduction into SAM thermal stability and SAM application in organic field effect transistors (OFETs) can be found e.g. in a Thesis by Daniel Käfer, Ruhr-University Bochum, 2008, http://www-brs.ub.ruhr-uni-bochum.de/netahtml/HSS/Diss/KaeferDaniel/diss.pdf, particularly on pages 130-162. Attempts to prepare electronic devices with an oriented layer of dipole molecules on the interface between an inorganic electrode and an adjacent organic layer are known also from patent literature, e.g., from WO2012/001358 and the documents cited therein. Despite the steady progress in the field, there is still an unmet demand for highly effective and stable hole injecting layers.
Only very few examples of molecular layers comprising true strong electrical p-dopants that could create holes e.g. in triarylamines (compound class that is currently most widely used in organic electronic devices as hole transporting matrices) is known. Quinoid systems substituted with electron withdrawing groups (EWGs) like tetrafluoro-tetracyanoquinodimethane (F4TCNQ) or hexaazatriphenylene (HAT) derivatives substituted with EWGs like hexaazatriphenylene hexacarbonitrile (HATCN) were successfully vacuum deposited on noble metal surfaces and strong influence of these layers on the photoelectron spectra was experimentally proven. Nevertheless, it is not yet known whether those layers could be deposited also from solution and whether p-dopants like F4TCNQ or HATCN containing amine and/or nitrile groups having only weak Lewis basicity are anchored to the metal surface strong enough to sustain a solution processing of a further organic layer on top of the molecular charge injecting layer.
It is an object of the present invention to provide an electronic or optoelectronic device wherein the hole injecting layer overcomes disadvantages of the prior art, preferably devices wherein the hole injection layer comprises a strong electrical p-dopant capable to inject holes effectively into currently used triarylamine hole transporting matrices and wherein the hole injecting layer is anchored strong enough to sustain solution processing of an adjacent organic layer. Another object of the invention is to provide a process that enables the desired electronic or optoelectronic devices with a strongly anchored and effective hole injecting layer. Yet another object of the invention is to provide new compounds enabling the desired devices and their manufacture.